NEET MDS Lessons
Physiology
A rise in blood pressure stretches the atria of the heart. This triggers the release of atrial natriuretic peptide (ANP). ANP is a peptide of 28 amino acids. ANP lowers blood pressure by:
- relaxing arterioles
- inhibiting the secretion of renin and aldosterone
- inhibiting the reabsorption of sodium ions in the collecting ducts of the kidneys.
The effects on the kidney reduce the reabsorption of water by them thus increasing the flow of urine and the amount of sodium excreted in it (These actions give ANP its name: natrium = sodium; uresis = urinate). The net effect of these actions is to reduce blood pressure by reducing the volume of blood volume in the system.
The Heartbeat
During rest, the heart beats about 70 times a minute in the adult male, while pumping about 5 liters of blood.
The stimulus that maintains this rhythm is self-contained. Embedded in the wall of the right atrium is a mass of specialized heart tissue called the sino-atrial (S-A) node. The S-A node is also called the pacemaker because it establishes the basic frequency at which the heart beats.
The interior of the fibers of heart muscle, like all cells, is negatively charged with respect to the exterior. In the cells of the pacemaker, this charge breaks down spontaneously about 70 times each minute. This, in turn, initiates a similar discharge of the nearby muscle fibers of the atrium. A tiny wave of current sweeps over the atria, causing them to contract.
When this current reaches the region of insulating connective tissue between the atria and the ventricles, it is picked up by the A-V node (atrio-ventricular node). This leads to a system of branching fibers that carries the current to all parts of the ventricles.
The contraction of the heart in response to this electrical activity creates systole.
A period of recovery follows called diastole.
- The heart muscle and S-A node become recharged.
- The heart muscle relaxes.
- The atria refill.
The Electrocardiogram
The electrical activity of the heart can be detected by electrodes placed at the surface of the body. Analysis of an electrocardiogram (ECG or EKG) aids in determining, for example, the extent of damage following a heart attack. This is because death of a portion of the heart muscle blocks electrical transmission through that area and alters the appearance of the ECG
Control of the Heart
Although the A-V node sets the basic rhythm of the heart, the rate and strength of its beating can be modified by two auxiliary control centers located in the medulla oblongata of the brain.
- One sends nerve impulses down accelerator nerves.
- The other sends nerve impulses down a pair of vagus nerves
Accelerator Nerves
The accelerator nerves are part of the sympathetic branch of the autonomic nervous system, and like all post-ganglionic sympathetic neurons release noradrenaline at their endings on the heart.
They increase the rate and strength of the heartbeat and thus increase the flow of blood. Their activation usually arises from some stress such as fear or violent exertion. The heartbeat may increase to 180 beats per minute. The strength of contraction increases as well so the amount of blood pumped may increase to as much as 25-30 liters/minute.
Vigorous exercise accelerates heartbeat in two ways;
- As cellular respiration increases, so does the carbon dioxide level in the blood. This stimulates receptors in the carotid arteries and aorta, and these transmit impulses to the medulla for relay by the accelerator nerves to the heart.
- As muscular activity increases, the muscle pump drives more blood back to the right atrium. The atrium becomes distended with blood, thus stimulating stretch receptors in its wall. These, too, send impulses to the medulla for relay to the heart.
Distention of the wall of the right atrium also triggers the release of atrial natriuretic peptide (ANP) which initiates a set of responses leading to a lowering of blood pressure
The Vagus Nerves
The vagus nerves are part of the parasympathetic branch of the autonomic nervous system. They, too, run from the medulla oblongata to the heart. Their activity slows the heartbeat.
Pressure receptors in the aorta and carotid arteries send impulses to the medulla which relays these by way of the vagus nerves to the heart. Heartbeat and blood pressure diminish.
The Body Regulates pH in Several Ways
- Buffers are weak acid mixtures (such as bicarbonate/CO2) which minimize pH change
- Buffer is always a mixture of 2 compounds
- One compound takes up H ions if there are too many (H acceptor)
- The second compound releases H ions if there are not enough (H donor)
- The strength of a buffer is given by the buffer capacity
- Buffer capacity is proportional to the buffer concentration and to a parameter known as the pK
- Mouth bacteria produce acids which attack teeth, producing caries (cavities). People with low buffer capacities in their saliva have more caries than those with high buffer capacities.
- Buffer is always a mixture of 2 compounds
- CO2 gas (a potential acid) is eliminated by the lungs
- Other acids and bases are eliminated by the kidneys
SPECIAL VISCERAL AFFERENT (SVA) PATHWAYS
Taste
Special visceral afferent (SVA) fibers of cranial nerves VII, IX, and X conduct signals into the solitary tract of the brainstem, ultimately terminating in the nucleus of the solitary tract on the ipsilateral side.
Second-order neurons cross over and ascend through the brainstem in the medial lemniscus to the VPM of the thalamus.
Thalamic projections to area 43 (the primary taste area) of the postcentral gyrus complete the relay.
SVA VII fibers conduct from the chemoreceptors of taste buds on the anterior twothirds of the tongue, while SVA IX fibers conduct taste information from buds on the posterior one-third of the tongue.
SVA X fibers conduct taste signals from those taste cells located throughout the fauces.
Smell
The smell-sensitive cells (olfactory cells) of the olfactory epithelium project their central processes through the cribiform plate of the ethmoid bone, where they synapse with mitral cells. The central processes of the mitral cells pass from the olfactory bulb through the olfactory tract, which divides into a medial and lateral portion The lateral olfactory tract terminates in the prepyriform cortex and parts of the amygdala of the temporal lobe.
These areas represent the primary olfactory cortex. Fibers then project from here to area 28, the secondary olfactory area, for sensory evaluation. The medial olfactory tract projects to the anterior perforated substance, the septum pellucidum, the subcallosal area, and even the contralateral olfactory tract.
Both the medial and lateral olfactory tracts contribute to the visceral reflex pathways, causing the viscerosomatic and viscerovisceral responses.
Vital Capacity: The vital capacity (VC) is the maximum volume which can be ventilated in a single breath. VC= IRV+TV+ERV. VC varies with gender, age, and body build. Measuring VC gives a device for diagnosis of respiratory disorder, and a benchmark for judging the effectiveness of treatment. (4600 ml)
Vital Capacity is reduced in restrictive disorders, but not in disorders which are purely obstructive.
The FEV1 is the % of the vital capacity which is expelled in the first second. It should be at least 75%. The FEV1 is reduced in obstructive disorders.
Both VC and the FEV1 are reduced in disorders which are both restrictive and obstructive
Oxygen is present at nearly 21% of ambient air. Multiplying .21 times 760 mmHg (standard pressure at sea level) yields a pO2 of about 160. Carbon dioxide is .04% of air and its partial pressure, pCO2, is .3.
With alveolar air having a pO2 of 104 and a pCO2 of 40. So oxygen diffuses into the alveoli from inspired air and carbon dioxide diffuses from the alveoli into air which will be expired. This causes the levels of oxygen and carbon dioxide to be intermediate in expired air when compared to inspired air and alveolar air. Some oxygen has been lost to the alveolus, lowering its level to 120, carbon dioxide has been gained from the alveolus raising its level to 27.
Likewise a concentration gradient causes oxygen to diffuse into the blood from the alveoli and carbon dioxide to leave the blood. This produces the levels seen in oxygenated blood in the body. When this blood reaches the systemic tissues the reverse process occurs restoring levels seen in deoxygenated blood.
Hypoxia
- Hypoxia is tissue oxygen deficiency
- Brain is the most sensitive tissue to hypoxia: complete lack of oxygen can cause unconsciousness in 15 sec and irreversible damage within 2 min.
- Oxygen delivery and use can be interrupted at several sites
|
Type of |
O2 Uptake |
Hemoglobin |
Circulation |
Tissue O2 Utilization |
|
Hypoxic |
Low |
Normal |
Normal |
Normal |
|
Anemic |
Normal |
Low |
Normal |
Normal |
|
Ischemic |
Normal |
Normal |
Low |
Normal |
|
Histotoxic |
Normal |
Normal |
Normal |
Low |
- Causes:
- Hypoxic: high altitude, pulmonary edema, hypoventilation, emphysema, collapsed lung
- Anemic: iron deficiency, hemoglobin mutations, carbon monoxide poisoning
- Ischemic: shock, heart failure, embolism
- Histotoxic: cyanide poisoning (inhibits mitochondria)
- Carbon monoxide (CO) poisoning:
- CO binds to the same heme Fe atoms that O2 binds to
- CO displaces oxygen from hemoglobin because it has a 200X greater affinity for hemoglobin.
- Treatment for CO poisoning: move victim to fresh air. Breathing pure O2 can give faster removal of CO
- Cyanide poisoning:
- Cyanide inhibits the cytochrome oxidase enzyme of mitochondria
- Two step treatment for cyanide poisoning:
- 1) Give nitrites
- Nitrites convert some hemoglobin to methemoglobin. Methemoglobin pulls cyanide away from mitochondria.
- 2) Give thiosulfate.
- Thiosulfate converts the cyanide to less poisonous thiocyanate.
- 1) Give nitrites
Normal Chemical Composition of Urine
Urine is an aqueous solution of greater than 95% water, with a minimum of these remaining constituents, in order of decreasing concentration:
Urea 9.3 g/L.
Chloride 1.87 g/L.
Sodium 1.17 g/L.
Potassium 0.750 g/L.
Creatinine 0.670 g/L .
Other dissolved ions, inorganic and organic compounds (proteins, hormones, metabolites).
Urine is sterile until it reaches the urethra, where epithelial cells lining the urethra are colonized by facultatively anaerobic gram-negative rods and cocci. Urea is essentially a processed form of ammonia that is non-toxic to mammals, unlike ammonia, which can be highly toxic. It is processed from ammonia and carbon dioxide in the liver.
1. Automatic control (sensory) of respiration is in - brainstem (midbrain)
2. Behavioral/voluntary control is in - the cortex
3. Alveolar ventilation -the amount of atmospheric air that actually reaches the alveolar per breath and that can participate in the exchange of gasses between alveoli and blood
4. Only way to increase gas exchange in alveolar capillaries - perfusion-limited gas exchange
5. Pulmonary ventiliation not effected by - concentration of bicarbonate ions
6. Central chemoreceptors - medulla - CO2, O2 and H+ concentrations
7. Peripheral chemoreceptors - carotid and aortic bodies- PO2, PCO2 and pH
8. Major stimulus for respiratory centers - arterial PCO2
9. Rhythmic breathing depends on
1. continuous (tonic) inspiratory drive from DRG (dorsal respiratory group)
2. intermittent (phasic) expiratory input from cerebrum, thalamus, cranial nerves and ascending spinal cord sensory tracts
10. Primary site for gas exchange - type I epithelial cells for alveoli